Wonderflex

Wonderflex™ is a special material that consists of a thermoplastic and a supporting inlay. It can be easily formed with a heat source like a hot air pistol, oven or hot water. At 70°C (160°F) it becomes mouldable and static again as it cools down.

It's used mostly by Cosplayers for armor parts and wheapons. It comes in big sheets of 1000mm x 1450mm

Apparently Wonderflex seems largely unavailable outside of US. There is a importer in UK, which sells it at almost double price (as of Spring 2009). However, for small orders an eighth of a sheet (363x500mm) comes in handy.